1. Field of the Invention
The present invention relates to a compression-ignition type engine.
2. Description of the Related Art
In a usual compression-ignition type engine, fuel of a mean particle size of about 20 .mu.m to 50 .mu.m or less is injected into a combustion chamber after about 30 degrees before top dead center in the compression stroke. In such a compression-ignition type engine, part of the injected fuel is immediately vaporized just when the injection is begun. The succeeding fuel enters into the flame of combustion of the vaporized fuel and thus the injected fuel is successively burned. If the fuel entering into the flame of combustion is made to be successively burned in this way, however, the fuel will be burned in a state of air shortage, so a large amount of unburnt HC or soot will be generated.
In such a usual compression-ignition type engine, further, the fuel injection is formed in a limited region and therefore the combustion is performed in a limited region in the combustion chamber. If combustion is performed in such a limited region, however the local combustion temperature becomes higher than compared with the case where combustion is carried out in the entire interior of the combustion chamber, and accordingly a large amount of NO.sub.x is produced. Further, the smaller the mean particle size of the injected fuel, the greater the fuel vaporizing immediately upon injection, so the more severe the sudden pressure rise caused by the explosive combustion at the elapse of the ignition delay time after the start of the injection and as a result the higher the combustion temperature, so the still greater amount of NO.sub.x which is produced.
In this way, so long as the conventional combustion method is used, it is impossible to avoid the production of soot and NO.sub.x. Accordingly it is necessary to make fundamental changes to the combustion method in order to prevent the generation of soot and NO.sub.x.
Known in the art is a compression-ignition type engine wherein, in order to prevent the generation of the soot and NO.sub.x, fuel is conically injected from a fuel injector arranged in the combustion chamber toward the top face of the piston, the mean particle size of the fuel droplets of the injected fuel is made larger than a predetermined particle size at which the temperature of the fuel droplets reaches the boiling point of the main component of the fuel at about the top dead center of the compression stroke, which boiling point is determined by the pressure in the combustion chamber, and the fuel injection is carried out during a predetermined period from the start of an intake stroke to about 60 degrees before top dead center of the compression stroke (refer to European Patent Publication No. 0639710).
In this engine, by conically injecting the fuel from the fuel injector toward the top face of the piston during the period from the start of the intake stroke at which the pressure in the combustion chamber is low to about 60 degrees before the top dead center of the compression stroke, the injected fuel is made to diffuse in the combustion chamber. Further, in this engine, most of the fuel droplets reach the boiling point after the top dead center of the compression stroke and thus the vaporization of the fuel droplets is started all at once after the top dead center of the compression stroke. When the diffused fuel droplets are vaporized all at once after the top dead center of the compression stroke in this way, a sufficient amount of air exists at the periphery of the fuel droplets, so the generation of soot is prevented, and, since the combustion temperature does not become extremely high, the generation of NO.sub.x is prevented.
In this engine, if the spread angle of the injected fuel is made small, when the fuel injection is carried out near 60 degrees before top dead center, that is, when the fuel injection is carried out when the piston position is relatively high, the injected fuel impinges upon and adheres to the top face of the piston. Accordingly, in this engine, the spread angle of the injected fuel is made considerably large in order to avoid this. When the spread angle of the injected fuel is made large in this way, when the fuel injection is carried out when the piston position is relatively high, since the pressure in the combustion chamber is relatively high, the injected fuel diffuses well in the entire interior of the combustion chamber without reaching the inner circumferential surface of the cylindrical bore, but when the fuel injection is carried out when the piston position is low, since the pressure in the combustion chamber is low at this time, the reach of the injected fuel becomes long, and thus the injected fuel impinges upon and adheres to the inner circumferential surface of the cylindrical bore. As a result, there is not only a problem of generation of a large amount of unburnt HC, but also a problem that the fuel is mixed into the lubricant oil.